, Volume 10, Issue 3, pp 503-518
Date: 05 Apr 2007

Soil Nutrients Limit Fine Litter Production and Tree Growth in Mature Lowland Forest of Southwestern Borneo

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Efforts to improve models of terrestrial productivity and to understand the function of tropical forests in global carbon cycles require a mechanistic understanding of spatial variation in aboveground net primary productivity (ANPP) across tropical landscapes. To help derive such an understanding for Borneo, we monitored aboveground fine litterfall, woody biomass increment and ANPP (their sum) in mature forest over 29 months across a soil nutrient gradient in southwestern Kalimantan. In 30 (0.07 ha) plots stratified throughout the watershed (∼340 ha, 8–190 m a.s.l.), we measured productivity and tested its relationship with 27 soil parameters. ANPP across the study area was among the highest reported for mature lowland tropical forests. Aboveground fine litterfall ranged from 5.1 to 11.0 Mg ha−1 year−1 and averaged 7.7 ± 0.4 (mean ± 95 C.I.). Woody biomass increment ranged from 5.8 to 23.6 Mg ha−1 year−1 and averaged 12.0 ± 2.0. Growth of large trees (≥60 cm dbh) contributed 38–82% of plot-wide biomass increment and explained 92% of variation among plots. ANPP, the sum of these parameters, ranged from 11.1 to 32.3  Mg ha−1 year−1 and averaged 19.7 ± 2.2. ANPP was weakly related to fine litterfall (r = 0.176), but strongly related to growth of large trees at least 60 cm dbh (r = 0.848). Adjusted ANPP after accounting for apparent “mature forest bias” in our sampling method was 17.5 ± 1.2  Mg ha−1 year−1.Relating productivity measures to soil parameters showed that spatial patterning in productivity was significantly related to soil nutrients, especially phosphorus (P). Fine litterfall increased strongly with extractable P (r = 0.646), but reached an asymptote at moderate P levels, whereas biomass increment (r = 0.473) and ANPP (r = 0.603) increased linearly across the gradient. Biomass increment of large trees was more frequently and strongly related to nutrients than small trees, suggesting size dependency of tree growth on nutrients. Multiple linear regression confirmed the leading importance of soil P, and identified Ca as a potential co-limiting factor. Our findings strongly suggest that (1) soil nutrients, especially P, limit aboveground productivity in lowland Bornean forests, and (2) these forests play an important, but changing role in carbon cycles, as canopy tree logging alters these terrestrial carbon sinks.